Lithium secondary cell and method for manufacture thereof

ABSTRACT

A lithium secondary battery in the present invention includes as a negative electrode a lithium alloy, lithium metal formed on a conductive substrate by vacuum film-forming, or the lithium metal having a hydrophobic material layer formed on a surface of an amorphous metallic lithium. Such a lithium secondary battery gives satisfaction with no dendrite formation and long cycle life.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a battery using lithium metal asa negative electrode, specifically to a secondary battery with asuperior cycle life in which the growth of a dendrite can be suppressed.

BACKGROUND AND RELATED ART

[0002] A lithium battery using a lithium metal as a negative electrodehas excellent energy density. However, a dendrite grows on the surfaceof the lithium metal serving as a negative electrode upon charging, andthe dendrite may penetrate a separator and cause short circuit betweenthe negative electrode and the positive electrode, resulting in failureof the function of the battery. Additionally, the short circuit maycause an abnormal reaction, and, as a result, causes a problem on thesafety of the battery. The growth of the dendrite may deteriorate thecycle characteristics of the battery.

[0003] Hence, the method for preventing the dendrite growth has beenproposed that a lithium alloy is formed by mixing the lithium metal withother components such as aluminum, bismuth, lead, tin, and indium, or alithium oxide film is formed on the surface of the lithium metal.According to these methods, however, the operating voltage of thebattery is lowered and the energy density is smaller as compared with anegative electrode made of the lithium metal.

[0004] Moreover, Japanese Patent Laid-Open No. 7-296812 discloses theproposal, instead of using the lithium metal foil formed by rolling; theamorphous lithium or the amorphous lithium alloy is used for thenegative electrode.

[0005] The surface of the metallic lithium in an amorphous state of suchnegative electrode is made to be less likely to form a highly activereaction sites such as a crystal grain boundary, which is a singularpoint of the growth of dendrite. However, a battery with excellentcharacteristics cannot be obtained only by making an amorphous state inthe negative electrode.

[0006] An object of the present invention is to provide lithium metal ina secondary battery, which is stable, is inhibited to grow dendriteshape. Such a secondary battery shows a superior energy density and asuperior electromotive force upon long cycling.

DISCLOSURE OF INVENTION

[0007] Regarding a lithium secondary battery having a lithium metal or alithium alloy as a material in a negative electrode, the drawback isovercome by the present invention. In the present invention, the batteryhas the negative electrode comprising of the lithium metal or thelithium alloy formed on the conductive substrate, the lithium metal orthe lithium alloy formed by vacuum film-forming, or the amorphousmetallic lithium or the amorphous lithium whose surface is coated with ahydrophobic material layer.

[0008] Further, according to the above-mentioned lithium secondarybattery in the present invention, the hydrophobic material layercomprises at least a material selected from hydrocarbon and ester. Thecarbon atom in the hydrocarbon or the ester may be partially substitutedwith silicon atom, or the hydrogen atoms in the hydrophobic material maybe partially or entirely substituted with the fluorine atoms.

[0009] Additionally, according to the above-mentioned lithium secondarybattery in the present invention, the hydrophobic material layer isformed on 90% or more of the surface of the negative electrode.

[0010] According to the above-mentioned lithium secondary battery in thepresent invention, the ester is made of at least a material selectedfrom fatty ester, phenylcarboxylic acid ester, and diester. The carbonatoms in the ester may be silicon partially substituted with siliconatoms, or the hydrogen atoms in the ester partially may be partially orentirely substituted with the fluorine atoms.

[0011] According to the above-mentioned lithium secondary battery in thepresent invention, the hydrophobic material is at least a materialselected from the fluorine carboxylic acid ester, phthalate ester, andbenzoic ester. The silicon atoms may partially substitute for the carbonatoms in these esters, or fluorine may substitute for hydrogen partiallyor entirely in these esters.

[0012] According to the above-mentioned lithium secondary battery in thepresent invention, the hydrophobic material is a material such asdioctyl phthalate, cetylnaphthalene, carboxylic acid ester, fluorinecarboxylic acid ester, or neroli oil.

[0013] Further, according to the manufacturing method of the lithiumsecondary battery using lithium metal or a lithium alloy as a negativeelectrode in the present invention, the negative electrode is opposed toa positive electrode via a separator. The negative electrode is formedby the deposition of the lithium metal (or its alloy) or the amorphousmetallic lithium (or its alloy) on the conductive substrate by vacuumfilm-forming. The negative electrode may be coated with the hydrophobicmaterial by dipping in a solution contained with the hydrophobicmaterial, by the sputtering method or by the vapor deposition method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 shows a spectrum measured by an X-ray photoelectricspectrometry (XPS) on a surface of a negative electrode of the presentinvention.

[0015]FIG. 2 shows charge/discharge characteristics of a lithiumsecondary battery according to an example of the present invention.

EMBODIMENT OF THE INVENTION

[0016] It is found out in the present invention that for a regardingmetallic lithium or a lithium alloy that is used as a negative electrodeof a lithium secondary battery, or metallic lithium or the lithium alloythat is composed of an amorphous metal formed by vacuum film-forming,since a hydrophobic material layer is formed thereon, a stable surfacecan be achieved even when charging/discharging is repeated after thebattery is assembled, dendrite is not likely to occur, and favorablecharge/discharge characteristics such as cycle characteristics can beobtained.

[0017] The metallic lithium or lithium alloy of the present invention ismanufactured on a conductive body by a vacuum film-forming method suchas vacuum evaporation and a sputtering method, or is prepared by formingamorphous lithium or an amorphous lithium alloy on the conductive body.

[0018] The amorphous lithium and the amorphous lithium alloy are formedby suitable methods including a melting solution cooling method, aliquid rapid cooling method, an atomize method, a vacuum evaporationmethod, a sputtering method, a plasma CVD method, an optical CVD method,and a thermal CVD method.

[0019] Moreover, as the amorphous lithium alloy, it is possible to adoptan alloy of two elements or three or more elements that includes Li anda metal such as Al, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Sr, and Te,or the alloy further adding a material such as Si, Cd, Zn, and La or thelike.

[0020] Additionally, in the case of the lithium alloy, an alloy isadopted in which the content of components other than lithium is 70% orbelow in atomic ratio.

[0021] Furthermore, as the conductive substrate, it is possible to adopta sheet material such as metallic foil made of a material such ascopper, nickel, stainless, aluminum, and silver. The thickness is 5 to20 m.

[0022] Also, a lithium metal or a lithium alloy formed on the conductivesubstrate is preferably 10 to 500 m in thickness and is furtherpreferably 15 to 300 m in thickness.

[0023] As a material used for forming the hydrophobic material layer onthe surface of the lithium metal of the present invention, it ispossible to adopt at least a compound selected from carbon hydrogen andester that does not substantially react with lithium, silicon maypartially substitute for the carbon, and a compound is applicable inwhich fluorine may substitute for hydrogen partially or entirely.

[0024] Further, it is preferable that ester is at least a materialselected from fatty ester, phenylcarboxylic acid ester, and diester,silicon may partially substitute for the carbon, and fluorine maysubstitute for hydrogen partially or entirely. It is more preferablethat ester is at least a material selected from fluorine carboxylic acidester, phthalate ester, and benzoic ester, silicon may partiallysubstitute for the carbon, and ester is composed of a compound in whichfluorine may substitute for hydrogen partially or entirely.

[0025] To be specific, dioctyl phthalate, cetylnaphthalene, carboxylicacid, fluorine carboxylic acid ester, neroli oil and the like areapplicable. It is particularly preferable to adopt a compound havingaffinity on the surface of metallic lithium. A compound containing aphenyl group like dioctyl phthalate is particularly desirable.

[0026] Since surface treatment is performed on lithium metal by usingthe hydrophobic materials, a hydrophobic material film is formed on thesurface of lithium so as to improve the uniformity of the surface oflithium. Particularly a carboxylic acid group constituting ester has thefunction of bonding ester to the surface of lithium metal with stabilityand covering the surface of lithium metal with stability by using alkylchains and phenyl chains that are contained in ester.

[0027] Consequently, it is possible to suppress the growth of dendriteon the surface of lithium metal and to suppress the reaction withimpurities like moisture that is induced by a row material of anelectrolytic solution, a positive electrode and a separator, and so onwhen assembling the battery.

[0028] Further, as the positive electrode used in the lithium secondarybattery of the present invention, it is possible to adopt a complexoxide of Li_(x)MO₂ (here, M represents at least one transition metal)such as Li_(x)CoO₂, Li_(x)NiO₂, Li_(x)Mn₂O₄, Li_(x)MnO₃, andLi_(x)Ni_(y)Co_((1-y))O₂, and a conductive material such as carbon blackand a binder such as polyvinylidene fluoride (PVDF) that are dispersedand kneaded with an agent such as N-methyl-2-pyrrolidone (NMP) and areapplied onto a substrate such as aluminum foil.

[0029] Moreover, according to the lithium secondary battery of thepresent invention, the battery can be manufactured as follows: thenegative electrode, which has a hydrophobic surface layer formed on thesurface of lithium metal or a lithium alloy, and the above-mentionedpositive electrodes are stacked via the separator, which is composed ofa porous film made of fluorocarbon resin and polyolefin such aspolypropylene and polyethylene in dried air or an inert gas atmosphere,or the stacked electrodes are wound and are mounted in a battery can, orthe electrodes are sealed with a flexible film and so on, which iscomposed of a layered product of synthetic resin and metallic foil.

[0030] Additionally, as the electrolytic solution, it is possible toadopt an electrolytic solution in which lithium salt such as LiPF₆ isdissolved in an organic solvent such as propylene carbonate, ethylenecarbonate, dimethyl carbonate, and diethyl carbonate.

[0031] Also, instead of the above electrolytic solutions, a solidelectrolyte and a polymer electrolyte are applicable.

EXAMPLES

[0032] Referring to an example and a comparative example, the followingwill describe the present invention.

Example 1

[0033] (Manufacturing of Lithium by Vapor Deposition)

[0034] A vacuum vessel is decompressed in degree of vacuum to 10⁻⁵ Pa,copper foil with a length of 50 mm, a width of 50 mm, and a thickness of10 m is used as a substrate, lithium is evaporated by electron beam, andlithium with a thickness of 20 m is precipitated while setting atemperature of copper foil at 90. Subsequently, the substrate on whichlithium is evaporated is dipped into dioctyl phthalate in the vacuumvessel at 45 for three hours under a reduced pressure to form a surfacelayer made of dioctyl phthalate on a surface of lithium.

[0035]FIG. 1 shows an X-ray photoelectric spectrometry (XPS) spectrum onthe surface of the obtained lithium. A peak (285.0 eV) indicatinghydrocarbon exits and metallic lithium prepared by rolling and so ondoes not have a typical peak of lithium carbonate (290.2 eV).

[0036] Further, the surface layer, which is formed on the lithiumsurface and is reformed by dioctyl phthalate, is 7 nm in thicknessaccording to the measurement conditions.

[0037] Moreover, a peak area shows that 90% or more of the surface layeris reformed by dioctyl phthalate.

[0038] (Manufacturing of the Battery)

[0039] Lithium previously formed on the copper foil is cut in size of 45mm×40 mm, a nickel tab is welded to serve as a lithium negativeelectrode, Li_(x)Mn₂O₄ is mixed with carbon black and polyvinylidenefluoride (PVDF), and a positive electrode is formed by applying positiveelectrode paint, which is dispersed and kneaded whileN-metyl-2-pyrrolidone (NMP) serves as an agent, onto a surface ofaluminum foil so as to have a thickness of 130 m after drying.

[0040] The negative electrode and the positive electrode are stacked viaa separator made of polyethylene, and a laminate film is used as anouter casing material, in which a polypropylene film is stacked on asurface of the aluminum foil and a nylon film is stacked on the othersurface thereof, to form the lithium secondary battery.

[0041]FIG. 2 shows a result of a charge/discharge test conducted on theobtained lithium secondary battery under the following charge/dischargetest conditions.

[0042] (Charge/Discharge Test)

[0043] At temperatures of 20 and 45, a charging rate is set at 0.2 C and0.5 C, a discharging rate is set at 0.2 C, a charge end voltage is setat 4.2 V, and a discharge end voltage is set at 3.0 V. A depth ofdischarge (DOD) is 30%. FIG. 1 shows the relationship between adischarging capacity and the number of cycles that are obtained at 20and a charging rate of 0.2 C.

[0044] And then, in view of charge/discharge characteristics of FIG. 2,an average coulombic efficiency (%) E is calculated according to thefollowing equation.

E=100×{Q−Q _(ex)/(n−1)}/Q

[0045] Q denotes a capacity (Ah/g) upon charging and discharging, Q_(ex)denotes a capacity of excessive metallic lithium (Ah/g), and n denotesthe number of cycles until the excessive lithium is consumed completely.

[0046] Table 1 shows a test result obtained under each cycle testcondition. TABLE 1 Temperature Charging Coulombic (° C.) rate efficiency(%) Example 1 20 0.1 C 98 20 0.2 C 94 20 0.5 C 81 45 0.1 C 96 45 0.2 C85 45 0.5 C 79 Comparative 20 0.1 C 85 Example 1 20 0.2 C 80 20 0.5 C 7645 0.1 C 90 45 0.2 C 77 45 0.5 C —

COMPARATIVE EXAMPLE 1

[0047] After precipitating lithium metal by vacuum evaporation, abattery is formed in the same manner as Example 1 except that surfacetreatment is not performed using ester, and charge/discharge tests areconducted as in Example 1. The results are shown in Table 1.

INDUSTRIAL APPLICABILITY

[0048] The present invention makes it possible to achieve a lithiumsecondary battery possessing superior energy density, superiorelectromotive force, long cycle life, and good safety, which areobtained by using metallic lithium as a negative electrode.

1. A lithium secondary battery having a negative electrode comprising ametal selected from a lithium metal and a lithium alloy, the metal beingcoated with a hydrophobic material layer.
 2. A lithium secondary batteryhaving a negative electrode comprising a metal selected from amorphouslithium metal and an amorphous lithium alloy, the metal being coatedwith a hydrophobic material layer.
 3. The lithium secondary batteryaccording to claim 1 or 2, wherein the hydrophobic material layercomprises at least a compound selected from a group consisting of ahydrocarbon compound and an ester, and carbon atoms of the compound arepartially substituted with silicon atoms or hydrogen atoms of thecompound are partially or entirely substituted with fluorine atoms. 4.The lithium secondary battery according to claim 3, wherein the estercompound is at least a compound selected from a group consisting offatty ester, phenylcarboxylic acid ester and diester, and carbon atomsof the ester compound are partially substituted with silicon atoms orhydrogen atoms of the ester compound are partially or entirelysubstituted with fluorine atoms.
 5. The lithium secondary batteryaccording to claim 3, wherein the hydrophobic material is at least acompound selected from a group consisting of fluorine carboxylic acidester, phthalate ester, and benzoic ester, and carbon atoms of the estercompound are partially substituted with silicon atoms or hydrogen atomsof the ester compound are partially or entirely substituted withfluorine atoms.
 6. The lithium secondary battery according to claim 3,wherein the hydrophobic material is at least a compound selected from agroup consisting of dioctyl phthalate, cetylnaphthalene, carboxylic acidester, fluorine carboxylic acid ester, and neroli oil.
 7. Amanufacturing method of a lithium secondary battery having a negativeelectrode comprising a lithium metal or a lithium alloy, characterizedin that a surface of the lithium metal, the lithium alloy, or thelithium metal or the lithium alloy formed by vacuum film-forming, isprocessed by dipping into a hydrophobic material or by sputtering ahydrophobic material or by vapor deposition of a hydrophobic material,and then, the negative electrode is opposed to a positive electrode viaa separator to form a battery.
 8. A manufacturing method of a lithiumsecondary battery having a negative electrode comprising a lithium metalor a lithium alloy, characterized in that a surface of an amorphouslithium metal or an amorphous lithium alloy, is processed by dippinginto a hydrophobic material or by sputtering a hydrophobic material orby vapor deposition of a hydrophobic material, and then, the negativeelectrode is opposed to a positive electrode via a separator to form abattery.